While sandblasting is a nice way to go, it's not always practical or feasible. If you can't blast, do as you are doing, grind with a low grit (16 or less) angle grinder, then use a wire brush or drill mounted brush to clean out any existing depressions caused by rust.
Not familiar with the OSPHO stuff, but I used POR 15, probably similar. They are both nice in that they will react with any rust that does start to form, makes the timing much easier. Apply and sand. Repeat as necessary.
After that just smooth out with any epoxy/filler combo you like. Apply and sand, repeat, repeat, etc. Some fillers sand easier than others, choose wisely. It will take a few go-rounds to get it all looking good. After that apply a few coats of unfilled epoxy, hand sand between coats, then final sand up to 600 grit. Then paint with whatever you're using on the rest of the bottom.
It killed my knees crouching beneath the boat doing this. Wish I could have turned the boat upside down.